In synchronous digital data transmission systems it is necessary to sample received signals once during each signaling interval at uniformly spaced instants of time. Data decisions are made at sampling times. Overall error performance of a synchronous data transmission system depends critically on the choice of the sampling instant particularly when multilevel encoding is employed.
The sampling frequency used in a data receiver can be obtained generally from either the received data signal itself or from auxiliary transmitted frequencies, such as pilot tones. The sampling phase, however, should be obtained from the received signal alone since it has been subjected to phase distortion in traversing the transmission medium. Typically, both sampling frequency and phase have been extracted from the analog received wave by differentiation and rectification of its envelope and use of the resultant zero-crossing pulses to drive a tuned circuit or a phase locked oscillator loop. Diffferentiation and rectification can be broadly classified as nonlinear processes by means of which a discrete spectral line at the sampling frequency can be generated.
It is an object of this invention to improve upon and simplify timing recovery in digital data transmission systems which employ nonlinear networks to generate a discrete spectral line.
It is a further object of this invention to generate a timing recovery wave for digital data transmission systems without reference to any observation of, or operation on, the analog envelope of the received signal wave.
It is another object of this invention to recover an accurate timing wave in digital data systems by comparing successive periods of the received signal wave.
It is still another object of this invention to recover the timing wave from either a baseband or a modulated passband data signal.
It is yet another object of this invention to recover the timing wave for a modulated passband signal without interference from double-frequency signal components.